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Highway Design - Class notes 6 A

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					Class 6 – Design Speed –
          Part A
                         Speed
• Minimizing travel time – important concern
• Speed of vehicles depend on:
   – Vehicle operating capabilities
   – Driver capability
   – Road characteristics
   – Weather
   – Road conditions
   – Traffic volume
   – Posted speed limit
• Significant effect on safety (particularly in severity of
  collisions)
               Speed (contd.)
• Collisions rates affected by speed variations
• The features and dimensions for the design of a road
  should be appropriate to travel speed

Types:
• Design speed = selected speed used to determine
  geometric design features
• Operating speed = free flow (spot speed)
• Running speed = length of highway section ÷ running
  time
• Posted speed = speed limit
              Desired Speed
• A speed at which the driver wishes to travel,
  determined by a combination of motivation and
  comfort
• Motivation – length of trip
• Comfort – perception of different conditions
• Drivers adjust their speed to the physical
  limitations and the prevailing traffic conditions
              Design Speed
• Definition: the highest speed at which individual
  vehicles can travel safely on a road when:
  – Weather conditions are favorable
  – Traffic density is low
  – Pavement surface is in good conditions
• Speed selected as a basis to establish
  appropriate geometric design elements for a
  particular section of road
• Not posted speed and not operating speed (but
  always higher than both)
         Design Speed (contd.)
• Design elements affected by design speed
  –   Horizontal and vertical alignment
  –   Super elevation
  –   Sight distance
  –   Other related indirectly
• Design speed is used to coordinate the
  geometric features of the road
• Range of design speeds 30 km/h to 130 km/h in
  intervals of 10 km/h
       Design Speed (contd.)
• Consistent with road function
• Freeways – highest design speed
   – 85th percentile speed
• Try to use as high of design speed as practical
  reasonable considering function of road,
  efficiency, safety, costs, environmental
  sensitivities, budget
• Logical to the topography
       Design Speed (contd.)
• Constant through a section
• Design speed may change along a road. The
  change should be made over a sufficient
  distance.
• The change can include advisory and regulatory
  signs
• See Table 4.2 in the next slide for Typical
  Minimum Design Speeds for Various Types of
  Highways (in mph). From Flexibility of Highway
  Design from FHWA
           Operating Speed
• Speed at which a driver is observed operating a
  vehicle
• Lower than desired speed
• Characteristics affecting this speed:
   – Access points
   – Commercial development
   – Road width
   – Number of lanes
     Operating Speed (contd.)
• Typically the 85th percentile speed of all vehicles
  at a particular location is used for design
  purposes
• Drivers typically adjust their speed based on
  knowledge of the road, posted speed,
  enforcement terrain, other vehicles, lane width,
  shoulder width, adjacent development, etc.
             Running Speed
• Definition: average or 85th percentile speed of all
  vehicles along a specific roadway as determined
  by the distances and the running times between
  two selected points
• Use as measure of the service of a road
• Varies according to the prevailing conditions
  according to hour of day
              Posted Speed
• Speed control, aimed at encouraging drivers to
  travel at an appropriate speed for prevailing
  conditions
• Typically set below the design speed to provide
  a latitude for driver exceeding the posted speed
• Selection is a traffic operations consideration as
  speed limitation for reasons of safety and
  economy
       Posted Speed (contd.)
• Design speed is related to the physical features
  of the road, cannot change without
  reconstruction
• Posted speed should be consistent with
  topography and adjacent land use
• Posted speed multiples of 10 km/h
• To be effective, it should be consistent with
  prevailing topographical and development
  conditions

				
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